Fix dropEvent bug (#828)

* Fix weakly referenced object no longer exists

Co-authored-by: Mohammed Ghannam <ghannam@zib.de>

* Update CHANGELOG

---------

Co-authored-by: Mohammed Ghannam <ghannam@zib.de>

Author: Joao-Dionisio

CHANGELOG.md

@@ -6,6 +6,7 @@
 - Add SCIP function SCIPgetTreesizeEstimation and wrapper getTreesizeEstimation
 - Add recipes sub-package
 ### Fixed
+- Fixed "weakly-referenced object no longer exists" when calling dropEvent in test_customizedbenders
 - Fixed incorrect writing/printing when user had a non-default locale
 ### Changed
 ### Removed

src/pyscipopt/scip.pxi

@@ -4861,6 +4861,8 @@ cdef class Model:
             _eventhdlr = SCIPfindEventhdlr(self._scip, n)
         else:
             raise Warning("event handler not found")
+        
+        Py_INCREF(self)
         PY_SCIP_CALL(SCIPcatchEvent(self._scip, eventtype, _eventhdlr, NULL, NULL))
 
     def dropEvent(self, eventtype, Eventhdlr eventhdlr):
@@ -4871,6 +4873,8 @@ cdef class Model:
             _eventhdlr = SCIPfindEventhdlr(self._scip, n)
         else:
             raise Warning("event handler not found")
+        
+        Py_DECREF(self)
         PY_SCIP_CALL(SCIPdropEvent(self._scip, eventtype, _eventhdlr, NULL, -1))
 
     def catchVarEvent(self, Variable var, eventtype, Eventhdlr eventhdlr):

tests/test_event.py

@@ -10,8 +10,9 @@ def eventinit(self):
         calls.append('eventinit')
         self.model.catchEvent(self.event_type, self)        
 
-    #def eventexit(self):
-    #    self.model.dropEvent(self.event_type, self)
+    def eventexit(self):
+        # PR #828 fixes an error here, but the underlying cause might not be solved (self.model being deleted before dropEvent is called)
+        self.model.dropEvent(self.event_type, self)
 
     def eventexec(self, event):
         assert str(event) == event.getName()